Supercritical fluid extraction (SFE) is a sample preparation technique used to extract analytes of interest from a sample, for example, environmental pollutants from a soil sample. Some types of sample preparations must be performed for a wide range of environmental, food, polymer, petroleum, pharmaceutical and other classes of samples due to the complex nature of the sample. Many of these samples are so complex that they cannot be directly evacuated by analytical techniques such as gas chromatography (GC) or liquid chromatography (LC). The complex samples must first go through a sample preparation step to perform a gross separation of the analytes of interest from the matrix in which they are contained, for example, the environmental pollutants from soil. After the sample preparation step, then just the analytes of interest are analyzed by the analytical technique.
The most popular sample preparation steps are Soxhlet extraction and liquid extraction. An alternative to these types of extraction techniques is supercritical fluid extraction (SFE). SFE offers a relatively rapid, simple and inexpensive technique to perform sample preparations. The basis of SFE is that a fluid, such as carbon dioxide, is held at a specific pressure, temperature and flow rate, which is above its critical temperature and pressure and thus is a supercritical fluid, is passed through the matrix containing the analytes of interest. This matrix is contained in an extraction vessel. The fluid diffuses into the pores of the matrix, solubilizes the analytes of interest, and then carries the analytes away from the matrix. The analytes are then collected by some device, so that the analytes can be analyzed by some further analytical technique, such as chromatography. The matrix (now without analytes) is left behind in the extraction vessel. Supercritical fluids have favorable diffusivities and viscosities providing for good mass transfer characteristics. Their strength can be easily controlled, and since most are gases at ambient conditions. These are but a few of the advantages of supercritical fluid extraction.
Typically, an SFE system is comprised of a pump which pumps the supercritical fluid to an extraction vessel where analytes are extracted from a sample matrix. The analytes are then transported to a collection device and the supercritical fluid is depressurized to ambient pressure and is vented. The analysis of the collected analyte can be either "off-line", that is, remote from the collection device, or "on-line", that is, fluidically connected to the collection device. Overall "on-line" collection is a more simple collection method than off-line collection. The focus of the present invention is improvements in on-line SFE-chromatography collection and coupling techniques.
In the prior art, gas chromatographs have been connected on-line to SFE through the use of a transfer line with a restrictor at the end of the transfer line. The restrictor is placed through the septum of the GC inlet port and the analytes of interest are directly deposited into the GC inlet port which can be either a split-splitless GC injector or an on-column GC injector. The problem with this approach is 1) flow from the SFE is limited to about 0.3 ml/min. (liquid CO.sub.2 equivalent) maximum flow, above this flow the collection capability of the method is overloaded, 2) when using an on-column injector, flow rates above 0.1 ml/min. (liquid CO.sub.2 equivalent) usually causes the GC flame dector flame to blow out, 3) overall SFE sample size is limited due to the low flow rates. See J. M. Levy, E. Storozynsky, M. Ravey, "The Use of Alternative Fluids in On-Line Supercritical Fluid Extraction-Capillary Gas Chromatography", Journal of High Resolution Chromatography, Vol. 14, 1991, pp. 661-666; J. M. Levy, J. P. Guzowski, W. E. Huhak, "On-Line Multidimensional Supercritical Fluid Chromatography/Capillary Gas Chromatography", Journal of High Resolution Chromatography & Chromatography Communications, Vol. 10, 1987, pp. 337-341; and J. M. Levy, A. C. Rosselli, E. Storozynsky, R. Ravey, L. A. Dolata, M. Ashraf-Khorassani, "Supercritical Fluid Extraction Coupled to Gas and Supercritical Fluid Chromatography", LC-GC, The Magazine of Separation Science, Vol. 10, No. 5, May 1992, pp. 386-391.
Further examples of supercritical fluid extraction devices can be found in "Hernan J. Cortes, et al., On-Line Coupling of Supercritical Fluid Extraction with Multidimensional Microcolumn Liquid Chromatography/Gas Chromatography, Anal. Chem. 1991, 63, 2719-2724" and "J. H. Raymer, et al., Development of a Flexible, On-Line Supercritical Fluid Extraction--Gas Chromatographic (SFE-GC) System, Journal of Chromatographic Science, V29, November 1991, pp. 467-475."
The present invention has advantages over the prior art approach in the following ways: 1) since the collection step happens external to the GC inlet port, much higher flow rates of supercritical fluid can be used. For example, 3-5 ml/min. (compressed) or higher CO.sub.2 flow rates can be used which allows for faster extractions, larger volume extraction vessels and much lower sensitivities. This results in the fact that more matrix can be easily extracted allowing low part per billion and even potentially part per trillion extractions to be performed, 2) since the collection step is decoupled from the GC injection port, the collection trap can be packed with other types of packing (C.sub.18, etc.) allowing additional selectivity to be employed, and 3) since the collection step is decoupled from the GC injection port, the GC can be operated separately from the SFE system, allowing overall faster time of on-line SFE-chromatography analysis. Thus, the GC can be analyzing a sample while the SFE system is extracting the next sample.
In addition to the above benefits for gas chromatography, other benefits exist for liquid chromatography. Very little work has been done in the prior art to couple SFE to liquid chromatography due to the difficulties in the coupling of the techniques. Cortes, et al. shows one of the few approaches to SFE-LC coupling. This invention has advantages over the SFE-LC prior art in the following ways: 1) the collection step is decoupled from the LC injection step, thus the LC can be operated separately from the SFE system, allowing overall faster time of on-line SFE-LC analysis, 2) the collection means is separate from the LC column and of larger volume and thus capable of collecting a larger mass of analyte. This allows greater sensitivity in the extraction and also allows the use of a larger extraction vessel.